1. Field
Example embodiments relate to memory cell programming, and more particularly, to a memory cell programming method in which some of a plurality of bits may be programmed and then a stabilization voltage having an electric field opposite in polarity to an electric field of a programming voltage used in the programming of the bits may be applied to a memory cell.
2. Description of the Related Art
Electrically erasable and programmable non-volatile memory devices may retain data even when no power is supplied thereto. Flash memories may be representative of these memory devices.
Flash memories may store data according to charge storage. Each of a plurality of memory cells included in a flash memory may be a cell transistor that may include a control gate, a charge storage layer, a source, and a drain. Flash memories may change the value of data stored in a memory cell by controlling the amount of charge stored in a charge storage layer.
Cell transistors of flash memories may control the amounts of charge stored in charge storage layers according to a Fowler-Nordheim (F-N) tunneling mechanism. Data of a cell transistor may be erased by applying a ground voltage to a control gate of the cell transistor and applying a voltage higher than a power supply voltage to a semiconductor substrate (or a bulk layer). Under these erasure bias conditions, a strong electric field may be formed between a charge storage layer and a bulk layer due to a difference between voltages of the charge storage layer and the bulk layer. Consequently, charges existing in the charge storage layer may be emitted to the bulk layer due to an F-N tunneling effect. At this time, a threshold voltage of the cell transistor from which the data may be erased may decrease.
A cell transistor may be programmed by applying a voltage higher than a power supply voltage to a control gate and applying a ground voltage to a drain and a bulk layer. Under these bias conditions, charges may permeate into a charge storage layer of the cell transistor due to an F-N tunneling effect. Accordingly, a threshold voltage of the cell transistor may increase.
A state in which a threshold voltage of a cell transistor is negative due to the existence of negative charges in a charge storage layer of the cell transistor may be referred to as an erasure state. A state in which a threshold voltage of a cell transistor is greater than 0 due to permeation of charges into a charge storage layer of the cell transistor may be referred to as a program state.
It may take a predetermined period of time for charges injected into a charge storage layer to be distributed evenly in the charge storage layer. In other words, it may take a predetermined period of time for a threshold voltage of a cell transistor to become constant (i.e., for the threshold voltage to become saturated (“constant” may not necessarily mean “saturated”)) from when charges are initially injected into a charge storage layer.
Thus, at a time when a threshold voltage of a cell transistor is measured after charges are injected into a charge storage layer included in the cell transistor, the level of the measured threshold voltage may vary, and accordingly, the value of data stored in a corresponding memory cell may also vary. Additionally, when the threshold voltage of the cell transistor is measured before the threshold voltage becomes constant, an accurate data value may not be read out.